The present invention describes a coating solution for producing glassy layers on a substrate, and also a process for producing such coatings.
Processes for producing glassy layers from solutions of polyorganosiloxanes have been known for a long time. If a solution of this sort contains one or more dopants, for example, elements belonging to Group III and Group V of the periodic system, which are normally used as free acids, as oxide hydrates, as alkoxides or as oxides, the glassy layers produced from the solution can be used for doping semiconductor substrates.
But glassy layers without dopants are used as gettering layers for removing undesirable impurities from semiconductor substrates. They are also used as insulating layers, for example, in semiconductor technology or in the production of liquid crystal displays. In addition, it is known that glassy layers without dopants can be used as planarization layers in the production of semiconductor components, and as plasma-etch-resistant intermediate layers in multilayer systems for producing highly resolved patterns.
In all these applications, a uniform layer thickness has to be ensured, since a nonuniform layer thickness can considerably impair the insulating and masking functions of the layer. If a nonuniform layer is etched, for example, undesirable variations may occur in the dimensional fidelity of the pattern transfer. In the case of plasma etching processes, the intensity of the attack on the substrate situated beneath the layer is variable, so that variations in layer thickness may be transferred to the substrate.
If a glassy layer produced with a conventional coating solution is coated with a photoresist layer, a nonuniformity in the glassy layer (spin-on glass layer) may result in variations in exposure when the resist layer is exposed to monochromatic light via a projection process. Consequently, exposure errors in the resist can arise because the interferences produced in the thin glassy layer result, due to the varying thickness of the layer, in reflection differences.
If a spun-on glassy layer is used as a diffusion source for dopant material, variations may arise in the doping of a semiconductor material by virtue of nouniformity in the thickness of the layer.
The polyorganosiloxanes contained in glassy layers may be produced in a number of ways, in accordance with the varying nature of the coating solution. For example, U.S. Pat. No. 3,615,943 describes coating solutions that contain reaction products of silicon tetrachloride and acetic anhydride. The disadvantage of these products is, on the one hand, the expensive production associated with the starting materials mentioned and, on the other hand, the high temperature (at least 225.degree. C.) necessary to produce a glassy layer. No statements are made in the aforementioned patent on the stability of the coating solution, or on the uniformity of the surface of the resulting layer. The layer optionally may contain dopant material comprising the elements B, P, As, Sb, Cd, In, Ga and Al.
Reaction products that are produced in ethanol from tetraethyl orthosilicate, with the use of acetic anhydride and a dopant material comprising the elements B, P, As, Au and Zn as glass-forming components, are described in U.S. Pat. Nos. 3,837,873 and 3,915,766. Nothing is said in either patent as to the coating uniformity.
U.S. Pat. No. 4,152,286 also describes a coating solution in which tetraethyl orthosilicate reacts with acetic anhydride, but in a solvent mixture comprising ethoxyethanol and diethyl phthalate. It is argued that, in contrast to the solutions disclosed in U.S. Pat. Nos. 3,837,873 and 3,915,766, respectively, formation of a precipitate (crystallization of B.sub.2 O.sub.3 dopant, shortly after the "spin-on" step can be avoided by the use of this solvent mixture. No statement is made on storage stability or layer quality.
Since the content of high-boiling acetic anhydride is very high in the solutions disclosed in all the documents hitherto discussed, baking out at high temperatures is unavoidable.
Particularly uniform layers on semiconductor substrates are said to be obtained according to German Offenlegungsschrift No. 2,340,111 (corresponding to U.S. Pat. No. 3,789,023). Approximately 3% of glycerol is added to adjust the viscosity of dopant-containing solutions containing tetraethyl orthosilicate in ethanol and ethyl acetate. Alcohols higher than ethanol are not mentioned as solvents. No statement on the storage stability of the coating solution is included.
According to German Offenlegungsschrift No. 2,338,079 (corresponding to U.S. Pat. No. 3,832,202), a further improvement in the surface uniformity is obtained with mixtures comprising vinyltrichlorosilane reacted with ethanol and tetraethyl orthosilicate dissolved in aqueous ethanol to which glycerine has been added. But this approach is described as entailing a two-component system only, i.e., the mixture of the various components is indicated not to be storage stable.
Coating solutions for doping semiconductor substrates are described in German Offenlegungschriften No. 2,447,204; No. 2,530,574; No. 2,920,673; No. 2,952,116 and No. 3,030,298. These solutions contain tetralkyl orthosilicate, preferably tetraethyl orthosilicate, and an aqueous solution of a dopant in an alcohol, preferably methanol or 2-propanol. Furthermore, higher boiling components like polypropylene glycol may be added, as described in German Offenlegungsschrift No. 2,920,673. None of the cited patent documents refers to the shelf life of the solutions; nor is any mention made of higher alcohols as solvents.
U.S. Pat. No. 3,928,225 describes a boron-containing doping solution containing a tetraethyl orthosilicate, a metal salt (Ni, Pb, Ca, Sn), an organic acid in ethanol and ethylene glycol monomethyl ether or ethylene glycol, in which the polymerization temperature of the silicic acid ester can be reduced by adding lactic acid. No statements are made in relation to shelf life or layer quality.
U.S. Pat. No. 4,243,427 mentions a solution that is highly doped with phosphorus. This solution is described, however, as a two-component system consisting of a solution of tetraethyl orthosilicate in methanol and aqueous/methanolic phosphoric acid (as Al(H.sub.2 PO.sub.4).sub.3), it being necessary to mix the two components for forming a coating and to process them immediately, due to their instability.
From German Offenlegungsschrift No. 2,922,900, it is known that coatings available hitherto may exhibit layer-thickness differences (so-called "striations"). The tetraethyl orthosilicate solution in acetone described in the cited patent document is polymerized by adding nitric acid. Addition of silicone oil is said to eliminate the "striations." The coating solution is described as a two-component system, with the implication of a short shelf life for the ready-to-use solution.
A coating solution which contains a prepolymer, poly(silsesquioxane), in addition to poly(ethoxysiloxane) or poly(methoxysiloxane) is described in European patent application No. 0 112 168. In this case, the uniformity of the coating is claimed to be achieved by the combination of the two polymers, while the use of poly(ethoxysiloxane) alone results in "striations." The solution described is subjected to a vacuum treatment to remove residues of acid, especially of hydrochloric acid, which is used as catalyst for the hydrolysis. In order to keep evaporation loss low, solvents with a boiling point higher than 110.degree. (1 bar) are disclosed. If the acid is not removed by vacuum treatment, the solutions gel within 30 days.
A coating solution based on poly(organosiloxane) is described in European patent application No. 167 854. Since the poly(organosiloxane) layer cannot be adequately cross-linked, it remains soluble in organic solvents and, as a rule, has to be converted superficially to SiO.sub.2 in an oxygen plasma in order to achieve the necessary solvent resistance. Otherwise there is the risk of the formation of undefined mixed layers if photoresists are spun onto the uncured layer.
German Offenlegungsschrift No. 3,247,173 describes a stable doping solution in which the glass-forming component is produced by reaction of tetraethyl orthosilicate with an acidic dopant in the presence of a catalytically active quantity of a strong mineral acid or a Lewis acid in an anhydrous solvent, preferably anhydrous ethanol or propanol. The water necessary for the hydrolysis of the tetraethyl orthosilicate has to be added in a quantity which is less than the stoichiometric quantity, since otherwise the material gels and, in addition, a precipitate is formed. The access of additional atmospheric water therefore has to be prevented in an expensive manner. No examples of dopant-free coating solutions are mentioned.
A coating solution which does not contain dopants is described in German Offenlegungsschrift No. 3,537,626. It is comprised of a mixture of a tetraalkyl orthosilicate, a vinyltrialkoxysilane or vinyltriacyloxysilane and/or a .gamma.-glycidoxypropyltrialkoxysilane in a solvent containing a low aliphatic alcohol and water, to which a catalytic quantity of a mineral acid is optionally added. The layers produced from this solution contain nonhydrolyzable organic residues and, as a consequence, are dissolved by organic solvents if the drying temperature is too low. If such a layer is part of a multilayer system, an oxygen-containing plasma has to be used for plasma-etching the layer in order to remove the organic residues. The photoresist layers situated on top are also attacked at the same time, so that dimensional fidelity of the pattern transfers is not possible. No statements are made in the cited patent document concerning the storage stability of the coating solution and/or the uniformity of the spun-on layer.